Radar level gauge systems for measuring the level of a liquid or other filling materials in a container are well-known, and such a system generally comprises a transmitter for transmitting a microwave signal towards the surface of the liquid, a receiver for receiving the microwave signal reflected against the surface of the liquid, and a signal processing device for calculating the level of the liquid in the container from the propagation time of the transmitted and reflected microwave signal. Such device has become more and more important, particularly for petroleum products such as crude oil and products manufactured from it. By containers is here meant large containers constituting parts of the total loading volume of a tanker, or even larger usually circular-cylindrical land-based tanks with volumes of tens or thousands of cubic meters.
One factor enabling a high accuracy for radar level gauging system is that the velocity of the radar waves usually is well known or equal to the velocity of light in vacuum (299 792 458 m/s) divided by the square root of the dielectric constant of the gas (close to 1.0006 for normal air). However, for some applications the dielectric constant is not known sufficiently accurate to obtain a desired high accuracy.
In one particular kind of radar-based device for gauging the level of a liquid in a container the microwave signal is transmitted, reflected and received through a waveguide, such as a vertical steel tube mounted within the container, which acts as a waveguide for the microwaves on their way to and from the liquid surface. However, a problem in such systems is that the gas above the surface of the liquid reduces the velocity of the microwaves. This velocity reduction may be accurately estimated, but only if the gas composition, temperature and pressure are known, which is normally not the case.
A solution to the above-discussed problem is presented in U.S. Pat. No. 6,915,689 by the same applicant. This document inter alia describes a method for pipe measurement where the dielectric constant of the gas has a negligible or very small influence on the distance seen as the measured value. The method is typically intended for LPG measurements where CTS accuracy is required and where the dielectric constant can vary in the range 1.00-1.02 (air Lo pressured propane), in contrast to ordinary radar methods where the wavelength in the used atmosphere is known and can be used as the standard. By CTS (Custody Transfer Safety) is understood systems with a very high accuracy, such as ±2 mm over 20 m distance, which are certified by for instance an authority in order to be allowed for official or commercial measuring use. As described in U.S. Pat. No. 6,915,689 the problem with the not sufficiently known dielectric constant can be solved, but at the expense that the accurate measurement will depend on the diameter of the pipe instead of on the velocity of the radar waves. The diameter of the steel pipe has enough stability (incl. a temperature correction) to be used as a standard but the initial determination of the diameter might present a problem including the fact that the diameter may vary slightly (say +/−0.5%) along the pipe. The pipe typically has a length of 25 m, provided in 6 m sections that are welded together. Thus, an exact estimation of the diameter is difficult.
When ordinary petroleum products are used, i.e. such that are fluent at usual temperatures, the gas in the tube is typically air. The nominal dielectric constant in air is 1.0006 with a typical variation of +−0.0001. The tank content would, however, increase the dielectric constant over that of air in case of evaporation of hydrocarbons etc. Such increase may be notable. Further, when to gauge the level in a container that contains a liquefied gas under overpressure the change in velocity is highly notable. Among the common hydrocarbon gases propane has among the highest dielectric constant causing about 1% velocity decrease at a pressure of 10 bar (corresponding to ∈=1.02). Such large discrepancy is in many applications, such as in custody transfer (CTS) applications, not acceptable. A higher accuracy, defined as custody transfer accuracy, is thus often needed. By the expression custody transfer (CTS) accuracy is herein meant an accuracy sufficient for a possible approval for custody transfer, which is a formal requirement in many commercial uses of level gauging. In terms of propagation velocity custody transfer accuracy may imply an accuracy in determination of the level in the range of about 0.005-0.05%.
U.S. Pat. No. 6,915,689 further discloses the use of two different propagation modes in order to more accurately estimate the pipe diameter, and to take corrective actions accordingly. Different modes propagate with different speed, which makes it difficult to make measurements when several modes are present. Therefore, several simultaneous modes are normally avoided in radar level gauging. However, in U.S. Pat. No. 6,915,689 use of several modes is considered in order to provide estimates on environmental conditions, and in particular the pipe diameter.
However, a problem with the solution taught by this document is that it requires a rather complex feeding system in the transmitter in order to provide the different modes of the microwave signals, which increases the system complexity, cost and difficulty of use.
It is therefore a need for a simpler and/or more cost effective solution, which still provides at least about the same degree of accuracy and reliability in regard of the measurement results.